The present invention relates to a method of manufacture of tubular bodies, particularly for preserved food cans, by rolling up a thin metal sheet (having a thickness typically not in excess of 0.5 mm) into a tube and welding the adjacent edges of the rolled sheet.
A conventional process for manufacturing tubular bodies for preserved food cans consists in rolling up a rectangular metal sheet to bring the two edges together, seaming the edges to one another and sealing the junctions by brazing with a tin-lead alloy.
This process has drawbacks: the overlapping of the opposite sides of the metal sheet constitutes a loss of material and creates a bead whose edges hinder the crimping of the bottoms of the can and which increases the number of rejects. The lead contained in the brazing material contaminates the food products.
Numerous attempts have been made to find a more satisfactory solution, particularly by electrically welding overlapping edge regions of the rolled up sheet. The use of brazing material is avoided and the amount of overlap may be decreased when elaborate continuous electric welding with wire-electrode on automatic machines is used. There remains however an overlap which constitutes a longitudinal bead.
It is an object of the invention to improve upon the prior apparatuses and methods for manufacturing tubular bodies having a low wall thickness.
A more particular object of the invention is to provide a method of forming tubular metal can bodies which do not exhibit a longitudinal bead.
A further object of the invention is to provide a method of forming tubular bodies for preserved food cans which includes a radial expansion step.
In general, the present invention contemplates a welding apparatus which comprises support means provided with a pair of elongated parallel clamping means actuatable for clamping two opposite edge regions of a metal sheet portion formed into a tube section, with corresponding confronting edges of the edge regions situated between the pair of clamping means next to each other and level to each other for forming a rectilinear junction line situated between the pair of clamping means, and for releasing said edge regions, said clamping means being adapted to cooperate with the sheet portion for clamping the latter with clamping forces which are substantially perpendicular to the edge regions without substantially forcing said edges into abutment, said clamping means forming the only means which holds the sheet material portions in edge-to-edge relation forming said junction line. The apparatus further comprises stationary guiding means operatively associated with said support means for guiding the latter along a path parallel to said junction line and laser beam generating means having, with respect to said support means, a location for directing a laser beam between said clamping means at a point in the path of said junction line to engage the sheet portion at the junction line formed by the edges thereof. Means operatively associated with the support means are adapted to move the latter with respect to said laser-beam generating means in a direction and with a speed causing the sheet portion to be welded progressively along said junction line.
Such an apparatus provides bodies without longitudinal beads which do not require addition of filler metal.
The weld which can be obtained with laser welding and with high quality electric welding is of such strength that the metal sheet rather than the weld fails when the can is subjected to excessive expansion.
A process according to another aspect of the invention makes use of that feature: that process includes the steps of rolling up a metal sheet of small thickness until the opposite edge portions of the sheet contact, welding said edge portions by a method which insures a resistance to tensile forces equivalent to that of the metal sheet and subjecting the tubular body to radial expansion.
The expansion step provides numerous advantages. If the tubular bodies must be transported before use, they may be subjected to the expansion operation on arrival, which reduces considerably the volume to be transported. The expansion allows starting with sheet metal having a thickness greater than that which it is desired to attain for the can. In particular, when a thickness from 0.15 to 0.16 mm can be accepted (cans which have not to withstand successively a high internal pressure and vacuum during a further sterilizing operation), one may start with conventional sheet metal from 0.2 to 0.3 mm thick, less costly than the 0.15 mm metal sheet because of the economy of a second rolling, during manufacture thereof. The expansion step may be carried on the same apparatus as welding or on a separate apparatus.
Welded seams, whatever their method of manufacture, may present micro-leaks which, in the case of cans for preserved foods, cause impairment of the products. Such micro-leaks are difficult to detect. But, when the body is subjected to expansion, the micro-leak results into tear easily discernible on inspection and this is an additional advantage.
For obtaining a high quality weld, the confronting edge portions of the metal sheet should be accurately positioned during laser welding, for avoiding overlap, radial offset and distortion which would result in small gaps between the edges.
For that purpose, the rolling and welding steps may be carried out by circulating a metal sheet having a length much in excess of the length of said tubular body into an external guide which rolls up said sheet until its opposite edges are in contact and said edges are pressed against a stationary internal core, the gap between the core and guide being selected for preventing overlapping of the edges and then through the location where the laser beam is focussed.
The core may occupy the whole cross section of the tubular body. However, a core may also be provided on which only the edge regions of the sheet will bear. The laser beam is advantageously focussed over a diameter less than 0.05 mm and in the thickness of the edge-to-edge joint.
This extremely fine focussing allows the heat required for melting the metal sheet to be released only there where it is required. The laser energy is used under the best conditions and the amount of heat in the metal is kept to a minimum. When tin-plate is used, focussing has the advantage of only volatilizing the tin over an extremely small width, and reducing the risks of subsequent corrosion. This process also makes possible to use black steel sheet, which is subsequently coated with varnish; iron sheet with an electrolytic coating of chromium or nickel; or iron sheet whose surface layer is slightly tin-alloyed. It may be reminded that varnish does not adhere firmly to the edge of the sheet iron when there is an overlap, with the risk of corrosion, particularly from inside in the case of cans for preserved foods.
The laser will be advantageously a continuous emission laser, although the use of a pulsed laser may be contemplated having a sufficiently high frequency for the welding to be continuous. Typically, the laser used will emit in the infrared. When it is desired to weld bodies of cans for preserved foods or drinks, a sheet 0.2 to 0.3 mm thick may be used and a minimum power density of 5000 kW/cm.sup.2 will be generally required in order to have an acceptable speed of advance. This result may particularly be reached with a CO.sub.2 laser.
The sides of the sheet must have a straightness tolerance compatible with the dimensions of the focal spot of the laser, i.e. in practice less than 0.05 mm, to avoid gaps between the contacting edges. The radial offset (in the direction of the thickness of the sheet) shall not exceed 20% of the thickness.
In another embodiment of the invention, the edges of an individual metal sheet used for manufacturing a tubular body are retained in contact during welding by claws against which the regions of the sheet close to the edges are retained by vacuum forces, said claws being located to retain the edges without exerting substantial abutting forces in the circumferential direction of the tubular body.
The invention will be better understood from the following description of particular embodiments of the invention. The description refers to the accompanying drawings.